Telegraphone



{ c. N. HICKMAN A 1,944,238

TELEGRAPHONE A Filed April 15,;1931 2 sneeshegc 1 I BVM..

ATTORNEY Patented Jan. 23,v 1934 A UNITED STATES PATENT ori-'ICE TELEGRAPHONE Clarence N. Hickman, Jackson Heights,` N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York i Application April 15, 1931. Serial No. 530,167 6 Claims. (Cl. 179-1002) Thisinventon relates to sound recording and remnant. is ze'ro or in other words to a. point reproducing systems and more particularly to such that if the demagnetizing force is removed telegraphones in which, as is well known, the sigthe material is left in an entirely or very nearly nals are iecorded by varying the magnetic contotally demagnetized condition, and While still dition 0f a mOVing magnetic body and are reunder the influence'of this demagnetizing force, 60

produced by causing such a magnetized member it is subjected to the flux representing the sigto vary the'inductioninasuitable translating denals to be recorded. In this manner the revice. magnetizing of the material is carried out on `.The object of the invention `is to provide av the substantially straight portion ofthe hysternovel method and apparatus of this. kind and in esis loop so that maximum variation in the 55 general to improve the operation of such a remagnetic condition of the vcompleted record may cording and reproducing system. be obtained with minimum distortion. Another While the telegraphone is an old and Well important advantage in working about lthis parknown arrangement for recording and reproducticular point of the curve is found in thefact ing sound it has never been used commercially that for this point the noise in reproduction due 70 to an extent comparable to the use of phonoto the condition of the record material is also graphs or film recording. There are a number a minimum. of reasons for this the most important of which )in important feature of the invention is the are probably the inherent defects in the appa.- method of recording at substantially constant ratus as heretofore proposed. In general the voluxwhereby the higher frequencies are repre- 75 ume level of the reproduced signals of previous' sented in the record by larger variation in magsystems was low, the frequency range covered netization than in systems proposed heretofore. was very limited, the extraneous noise Was ob- This not only extends the range of higher frejectionable and the distortion of the wave forms quencies which can be recorded without producwas often so great as to render the reproduction ing distortion from voverloading in the low fre- 30 almost unintelligible. The linear velocity requency portions of the record, but also increases quired for the record member, which was usualthe effectiveA volume range of the system in that ly of wire, to obtain even these results was as the higherfrequencies are recorded at a much much as six to ten feet per second so that the greater `margin over the noise level which is ormachine itself was exceedingly noisy and even dinarilyhighest in this part of the range. 85 somewhat dangerous in the event of breakage These and other features of the invention will of the Wire. be more clearly understood from the following In one o f its earliest forms a single recording detailed description and the accompanying drawmagnet and a small rapidly moving Wire Were ings in which:

used and later when.. recording magnets were Figs. 1 and 2 show one form of a machine suit- 90 placed on opposite sides of the wire they. were able for recording and reproducing sounds acordinarily spaced along the wire to set up 1oncording to this invention; gitudinal magnetization in it for it was believed Fig. 3 is a typical magnetization curve and a that if they were directly opposed, a recording portion of a hysteresis loop for the recording tape or Wire of considerable thickness was necmaterial used; and 95 essary to be capable of retaining the record. In Fig. 4 is a schematic of a circuit for the arother words it was believed that the magnetic rangement of Fig. 1 whereby sounds are recorded poles induced on opposite sides of the recordmaand reproduced by the same machine. terial should be as far apart as practical to Referring now to the drawings, particularly to minimize the demagnetizing eifect of the poles Figs. 1 and 2, the mechanism for moving the 100 of opposite polarity on each other. In recordrecord member willlirst be described. The maing it was common practice to record on the chine has been designed particularly with a view normal .magnetization curve of the record mato conserving space and involves a number ofl terial by subjecting it simultaneously toapolariznovel features which are more fully described 50 ing flux and a flux representing signals to be and claimed in my copending application, Serial 105 recorded. No. 530168, filed April 15, 1931. The record In accordance with the general feature of this member 11 is preferably a tape of any suitable invention the record material is lrst very highmagnetic material about .001 inch thick and ly magnetz'ed which incidentally erases any pre- .125 inch wide, the ends of which are secured to vious history. It is then demagnetized until the reels 12 and 13 respectively.4 'Ifo keep the ma- 11 tension on the tape unwinds it from the reel 13 chine small, these reels are rotatably mounted on the same drive shaft 14, and clutches and 16 are provided to engage alternately so that either reel may be the feed reel or the take-up reel.

Since it is essential to good quality that the speed of any given part of the tape passing the magnet be the same for recording and reproducing and free from minor irregularities, the mo'- tion of the tape during both processes is controlled by the tape-wheel 17 which is positively driven by the pulley 18 on the other side of the frame 19 by the belt 20 from pulley 21 on'the main shaft. The shaft is driven at a suitable speed through worm gearing 22 by a motnr of ,which only the shaft 23 is shown.

To enable the tape tofpass freely from one reel to the other thetape wheel is mounted on'the frame 19 in an inclined position as shown, and similarly inclined guiding pulleys 24, 25 are pravided between it and the reels 12 and 13. The pulley 18 is made large enough to serve also as a fly-wheel for damping out minor irregularities in speed or a separate fly-wheel may be provided as desired.- A

For recording,` the lever 54 is moved vto engage the friction clutch 15 and clutch 55 on the tape wheel shaft; the tape wheel pulls the tape between the magnets from right to left by friction with the tape-wheel 17 and is; wound up on reel 12. At this time clutch 16 is disengaged sothat against the slight friction of the reel on the shaft 14 which is turning in a clockwise direction as viewed in Figi 1 and therefore prevents the tape from unwinding faster than required. Between guiding pulley 25 and tape-wheel 17 the polarizing magnets 26 and the recording magnets 27 are mounted and on either sides of these magnets are suitable damping pads 28 for damping out rnechanical vibrations in the tape as it passes the magnets.

In Fig.l 4 the sounds to be recorded are picked up by a transmitter 30 and with the switch 31 in the upper or recording position, the corresponding'electrical variations will be transmittedto the amplifier 32, the output transformer 33 and finally to coils 34 of the magnets 27. These magnets also have depolarizing coils 35 through which circulates a current from battery 36. Similar coils 37 form the windings of the polarizing magnets 26. As the tape is moved from right to left as described above it is brought to a very high transverse magnetization by the flux set up in the pole pieces 38 of the magnets 26. .In this way, the tape is magnetized along the normal magnetization curve 39 of Fig. 3 to some value such as 40 on the curve, but as it passes away from the tip of the pole pieces 38. the induction decreases to some value 41 according to the retentivity of the particular recording material used. While this would normally indicate a material of low retentivity, it should be remembered that this curve'is .plotted for the transverse mag,- netization of a tape which is only 1 mil thick.

Any previous history of the tape is now erased and it approaches magnet 27 in a uniform magnetic condition. I As it passes the pole tips 57 of the magnets each element of the tape is successively subjected simultaneously to the iiux set up by the current in both the coils 34 and 35. The pole pieces of magnets 26 and 27 are preferably held in proper relation to the tape by spring members 58 which equalize the pressure exerted on the opposite sides of it. The depolarizing coils Aportion of the curve.-

35 are arranged so that the flux set up by them is in the opposite direction through the tape to the polarizing flux set up by magnet 26. The value of the depolarizing current is adjusted by rheostat 42 so that with no currentin coils 34 the tape will be completely demagnetized and remagnetized inthe opposite sense to some point 43 on the curve, such that as the tape passes away from the magnet it returns along the curve 44 to zero. However, when the coils 34 are carrying alternating current representing sounds to be recorded, the condition of each element of the tapel just as it leaves the pole tips will be represented by points along the curve 45 which lie between points such as 46 and 47, according to the magnitude and direction of the alternating signal flux in the coils 34. As the tape moves beyond the influence of the magnets 27, the induction in each element decreases along curves such as-48 and 49, all of which are 'in general parallel to curve 44 so that in its final state on the reel 12 the record is represented by variations in the magnetic condition of the tape, the limiting values -of which are indicated by' points 51 and 52 on the line of zero magnetizing force. By recording these signals as variations in mag netization from the condition represented by point 43, which corresponds to the completely demagnetized condition of the tape, it is found that the surface noise during reproduction is much less than when the record is made on any other. While recording can be done about some point on the curve other than 43 it will be apparent that any signal variations which carry the induction above the point 41 will not leave any permanent record in the tape since the induction will automatically fall to the value at 41 as soon as the tape passes beyond the recording magnets. In other words, systems of this kind record only half waves of the signal or,

very imperfectly, Whereas by the method of the present invention the entire signal is retained in the tape.

It has been stated above that the signals are recorded preferably at constant flux or constant current. By this expression, it is meant that sound waves of differing frequencies but of such intensity as to produce equal acoustic pressures on the sound pick-up device 30, will produce equal changes of flux in the recording magnets 27 and hence equal Variations in the magnetization of the tape. In circuits commonly used heretofore, the increasing impedance of the recorder coils with frequency resulted in signal-currents and hence recording flux which decreased with increasing frequency so that the higher frequencies were represented in the record by such small variations in magnetization that the noise level became comparable to the signals within the desired frequency range. In order to record at constant flux, the recorder circuit impedance must be substantially constant over the frequency range to -be recorded. A well known but rather inefficient way of obtaining constant impedance in a circuit is to make the resistance very largev as compared to the inductance. A very convenient and emcient way of approximating this condition is to correlate the impedances of the recorder coils 34 and the output circuit of the amplifier 32 so that they are matched at high frequencies. In this case the effect of the increasing inductive reactance of the coils is compensated by the increased power transfer as the signal frequencies approach the matching frequency. In the pres- I at best, record the positive halves of the waves iso ent invention, therefore, the variations in the magnetization of the record are substantially independent of frequency and hence the margin of the signal over the noise, which is greatest at high frequencies, is materially increased so that the effective range of the instrument is extended to frequencies which could not be recorded with previous systems.

When a record has been made according to the foregoing procedure the tape must, of course, be rewound before it can be reproduced so that in reproduction it passes the magnets in the same direction as in the recording process. The rewinding of the tape is preferably done at high speed to avoid unnecessary delay. The clutch lever 54. is moved inwardly to engage clutch 16 and at the same time to disengage clutch 55 between the tape-reel 17 and the fly-wheel pulley 18, so that the tape wheel turns freely on shaft 29. Under this condition the clutch 16 has little tendency to slip and the tape is quickly wound up on the reel 13. When this has been done the clutch lever is again moved to disengage clutch 16 and to engage clutches 15 and 55 andthe tape is moved from right to left between the magnets under the control of the tape wheel. as before. During reproduction, the switch 31 of Fig. 4 is closed downwardly so that a circuit is made 'from the coils 34: through amplifier 32 to the loud speaker 56. It will be noted that the polarizing coils 37 and the depolarizing coils 35 are then open-circuited. The movement of. the magnetized tape between the pole tips 57 of the magnets 27 will produce alternating ilux in the pole pieces which will generate in the coils 27, currents corresponding to the signals recorded.

While the variations in magnetization are constant with frequency, as explained above, it will be apparent that due to the longer wave lengths of the low frequencies the rate of change in ux in the pole pieces during reproduction will be less for the lower than for the higher frequencies and hence the lower frequencies would ordinarily be reproduced at less thanthe intensity of the originalsounds. This may be corrected conveniently to a large extent by designing the amplifier to have a low input impedance so that it matches the coils 27 in the lower part of the range. The reproducing circuit is therefore relatively more efcient for low frequencies so that the output of thev loud speaker 56 will be a good quality reproduction of the original sounds for all the frequencies recorded.

The general method described above will be found applicable to various recording arrange-l quency range at much lower noise level than with methods previously proposed and to accom plish these results with a record member moving at only 8V per second, or even less, so that several hours of conversation can be recorded on a reel of moderate size.

What is claimed is:

l. In a telegraphone the method of recording signals in a magnetic material which consists in highly magnetizing the material, reducing the magnetization to a point about mid-way along the substantially straight portion of the hysteresis loop and subjecting the material in this state to alternating flux representing the signals to be recorded.

' 2. In a telegraphone the method of recording signals in a magnetic material at high intensity relative to the noise level. which consists in highly magnetizing the material, reducing the magnetization to a point where the remnant in the absence of recorded signals is substantially zero jand subjecting the material in this state to alternating ux representing the signals to be recorded.

3. In a telegraphone the method of recording signals in a magnetic material at high intensity and without vsubstantial distortion which consists in highly magnetizing the material transversely, reducing the magnetization to a point Where the remnant in the absence of recorded signals is substantially zero and subjecting the material in this state to alternating signal flux. 4. In a telegraphone the method of recording signals in a magnetic materialwhich consists in bringing the material to a high transverse magnetization along the normal magnetization curve, reducing the magnetizing force to zero, applying suflicient negative magnetizing force to reduce the magnetization to the point at which if the negative magnetizing force is removed, the lnduction remaining in the material is substantially zero, and subjecting the material in this state to transverseA magnetization in accordance with the signals to be recorded.

members contacting opposite sides of the tape,

and means for magnetizing the members simulp taneously with an alternating signal ux substantially constant with the frequency for sounds of equal intensity, and with a depolarizing ux sufficient to reduce the magnetization to a point' '..35

where the remnant in the absence of signal ux is substantially zero.

CLARENCE N. HICKMAN. 

